Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Winter is the springtime of the believer: its days are short, so he fasts; its nights are long, so he stands (in prayer)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8456